It's a little more insidious in the US, because there is an informal speed buffer of something like "10% + 2mph over the limit," but it is not codified into law anywhere, at least AFAIK. In general, people are not harassed for slightly excessive speed, but if the officer doesn't like you, or is having a bad day, or is behind in his quota (excuse me, "performance guideline") for the month, he is perfectly within his authority to write you a ticket for doing 71 in a 70 zone.
Now, there are reasons for him not to do so; a rational judge would probably look critically on an officer that submitted several such citations, for example, but that would require one to contest the ticket in court, something one may be loath to do if one is far from home. A good attorney could probably make the calibration argument in front of the judge and win, but that would require not only contesting the ticket but hiring an attorney, which may cost more than paying a simple speeding ticket in the first place. One would also hope that a high rate of contested citations would reflect negatively on the performance of the officer in his performance review, but that's assuming a lot (including that there actually is a high rate of contested citations, and it's not just you).
When speeding in the US, therefore, one counts on the largesse of the officer, something not guaranteed to be available.